Tool bars for use in mounting cutting tools such as most of conventional boring and reaming bars are manufactured to be elongated and thin in order to machine a deep hole. Since this configuration leads to low bending strength and stiffness, a bending phenomenon is generated, a machining accuracy is lowered and chattering is produced on a surface to be machined due to its low natural frequency, upon performing cutting operations. Accordingly, upper thresholds of a cutting speed and a feed rate are low, and it is impossible to machine a deep hole. Therefore, in order to overcome such disadvantages, various shapes of cutting tools and tool bars for use in mounting the cutting tools have been developed, and many other tool bars for use in mounting cutting tools, which are made of tungsten carbide alloy and tool steel having very excellent stiffness and rigidity characteristics, have been also developed in advanced countries. However, since it is difficult to machine cemented carbide, there is a disadvantage in that manufacturing costs of a tool bar exhibiting optimum performance become very high. Further, since the natural frequency of a boring bar or a reaming bar is low due to high densities of tungsten carbide and tool steel, there is a limitation on improvement of the cutting speed of the bar.